Vertically center SCM restart dialog

Only update margins based on taskbar insets since app bounds include
status bar insets.

Fix: 269102159
Test: Manual using `adb shell dumpsys activity service
      SystemUIService WMShell compatui restartDialogEnabled 1`
Change-Id: I1e5648849277a94ddb8abf99d5a23b49a7b2a37c